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Unformatted text preview: 2240 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 33, NO. 12, DECEMBER 1998 An I/Q Active Balanced Harmonic Mixer with IM2 Cancelers and a 45 Phase Shifter Takafumi Yamaji, Member, IEEE , Hiroshi Tanimoto, Member, IEEE , and Hideyuki Kokatsu Abstract To reduce dc offset caused by local-oscillator-signal self-mixing in a direct-conversion receiver, an active balanced harmonic mixer with second-order intermodulation (IM2) can- celer was proposed, and a direct downconverter containing the mixer and a 45 phase shifter was fabricated on a BiCMOS chip. The dc offset caused by the self-mixing was measured and found to be smaller than the noise level. A second-order intercept point at the input of C 37 dBm was achieved with the IM2 canceler, which is suitable for direct conversion receivers for quadrature phase-shift keying signals containing an amplitude modulation component. Index Terms Direct conversion receiver, harmonic mixer, IM2 canceler, phase shifter, self-mixing. I. INTRODUCTION A DIRECT conversion receiver has been attracting atten- tion as a possible architecture for realizing a single-chip receiver , . However, it has two serious problems that need to be overcome. One is dc offset caused by self-mixing of the local oscillator (LO) signal and the other is second-order intermodulation (IM2). To overcome these problems, the authors proposed an active balanced harmonic mixer . The dc offset variation due to the self-mixing can be reduced down to its noise level with this harmonic mixer. After analyzing the self-mixing problem in Section II, the mixer structure and a simple operation model of the harmonic mixer are described in Section III, and a mathematical analysis is given in the Appendix. An even harmonic type of harmonic mixer produces no second-order distortion in principle because it has odd sym- metric nonlinearity. In practice, imperfection in circuit sym- metry and imbalance in differential signal result in a small second-order distortion, which may be problematic in appli- cations containing an amplitude-modulation component. These requirements for IM2 are specified by a second-order intercept point at the input (IIP2). For example, an IIP2 of 34 dBm or larger is required for the personal handyphone system (PHS) if direct downconverters are used. An IM2 canceler for active harmonic mixer is proposed in Section IV. The cancelation has little dependence on frequency, and readjustment is not necessary in more than 100-MHz band containing the whole PHS band (18951918 MHz). Manuscript received May 7, 1998; revised July 13, 1998. T. Yamaji and H. Tanimoto are with the Research and Development Center, Toshiba Corporation, Saiwai-ku, Kawasaki 210 Japan....
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